In a wireless communications network, each device needs to transmit information by using frequency resources. Frequency resources are also referred to as spectrums. Spectrums may include licensed spectrums and unlicensed spectrums. The licensed spectrums are dedicated frequency resources of some operators, and the unlicensed spectrums are frequency resources for public use in the wireless communications network. With development of communications technologies, an amount of information transmitted in the wireless communications network increases on a daily basis, a data throughput in the wireless communications network may be improved by preempting an unlicensed spectrum to transmit information, so as to better satisfy user requirements.
In other approaches, a Hybrid Automatic Repeat Request (HARQ) manner is usually used to transmit data. For example, after a first device sends data to a second device, the second device feeds back an Acknowledgement/Negative Acknowledgement (ACK/NACK) message to the first device. The first device determines, according to feedback information of the second device, whether to transmit the data again or transmit another version of the data to the second device until the data is correctly transmitted. This data transmission process is an HARQ process. An HARQ process for each time of data transmission has an HARQ process number, so as to distinguish between different HARQ processes. In a scenario in which data is transmitted by preempting an unlicensed spectrum, after a device successfully preempts a channel of the unlicensed spectrum, data may be sent at a random moment. As a result, the device may not send the data at a starting moment of a complete subframe, that is, a first subframe sent by the device may not be a complete subframe. In this case, how to determine an HARQ process number of this incomplete subframe is a problem to be solved urgently.